3. _Geological Evidence as to the Antiquity of Man._--No geological fact can now be more firmly established than the ascending progression of animal life, whereby from the early invertebrates of the Eozoic and Primordial series we pass upward through the dynasties of fishes and reptiles and brute mammals to the reign of man. In this great series man is obviously the last term; and when we inquire at what point he was introduced, the answer must be in the later part of the great Cainozoic or Tertiary period, which is the latest of the whole. Not only have we the negative fact of the absence of his remains from all the earlier Tertiary formations, but the positive fact that all the mammalia of these earlier ages are now extinct, and that man could not have survived the changes of condition which destroyed them and introduced the species now our contemporaries. This fact is altogether independent of any question as to the introduction of species by derivation or by creation. The oldest geological period in which any animals nearly related in structure to man occur is that named the Miocene, and no traces of man have as yet been found in any deposits of this age. All human remains known belong either to the Pleistocene or Modern. Now the Pleistocene was characterized by one of those periods of glacial cold which have swept over the earth--by one of those great winters which have so chilled the continents that few forms of life could survive them--and man comes in at the close of this cold period, in what is called the Post-glacial age. Some geologists, it is true, hold to an interglacial warm period, in which man is supposed to have existed, but the evidence of this is extremely slender and doubtful, and it carries back in any case human antiquity but a very little way. I have, in my "Story of the Earth and Man," shown reason for the belief, in which I find Professor Hughes, of Cambridge, coincides with me,[133] that the interglacial periods are merely an ingenious expedient to get rid of the difficulties attending the hypothesis of the universal glaciation of the northern hemisphere. But, though man is thus geologically modern, it is held that historically his existence on earth may have been very ancient, extending perhaps ten or twenty, or even a hundred times longer than the period of six or seven thousand years supposed to be proved by sacred history. Let us first, as plainly and simply as possible, present the facts supposed thus to extend the antiquity of man, and then inquire as to their validity and force as arguments in this direction.